histedit: add description about "histedit --outgoing" to command help

Before this patch, there is no explicit description that argument is
treated as the URL of the destination repository when "--outgoing" is
specified.

This patch adds description about "histedit --outgoing" to command
help of it.

hgext/histedit.py

@@ -451,12 +451,16 b" actiontable = {'p': pick,"
      ('f', 'force', False,
       _('force outgoing even for unrelated repositories')),
      ('r', 'rev', [], _('first revision to be edited'))],
-     _("ANCESTOR"))
+     _("ANCESTOR | --outgoing [URL]"))
 def histedit(ui, repo, *freeargs, **opts):
     """interactively edit changeset history
 
     This command edits changesets between ANCESTOR and the parent of
     the working directory.
+
+    With --outgoing, this edits changesets not found in the
+    destination repository. If URL of the destination is omitted, the
+    'default-push' (or 'default') path will be used.
     """
     # TODO only abort if we try and histedit mq patches, not just
     # blanket if mq patches are applied somewhere